Various communications systems use direct sequence spread spectrum modulation to transmit data between a transmitter and a receiver. Some of these systems use relatively short spreading codes to transmit the information such as IEEE 802.15.4 “Zigbee” or IEEE 802.11b “Wireless LAN”. In these communications systems, the information to be sent is coded in the polarity of the spreading code: a logical “1” is coded in the spreading sequence being sent in true polarity while a logical “0” is sent in inverse polarity. Common to these systems is the fact that the wanted signal may be below the thermal noise level at times, however, due to the spreading gain inherent in the de-spreading process the receiver will still be able to recover the transmit information.
There are applications where direct sequence spread spectrum methodology may be used that employ a relatively low duty cycle, i.e., the transmitter sends information only from time to time. An example may be a wireless long distance remote access system. Such a system would have the receiver scan or poll for messages from the transmitter regularly, however, given the low duty cycle of the messages to be sent, the receiver will not see any actual messages coming in from the transmitter during most of the time. In such a case, it may be useful to find out if there is any actual direct sequence spread spectrum signal present so as to either continue the reception process or abort the scan/poll process. Further, it would be useful to detect any loss of incoming signal while trying to receive a message after a working wireless communication link had initially been established between transmitter and receiver.